The present invention is related to a lens assembly and more particularly to a lens combination for connection to the optical fiber terminal head of an optical fiber fine displacement detecting system so as to extend the effective working distance between the detecting head of a detecting system and an object in detection.
The layout of an optical fiber fine displacement detecting system with which the present invention is concerned is illustrated in FIG. 4A. A bundle of optical fibers F is used as a detecting head, part of which is light emitting optical fibers F1 which transfer monochromatic light from light source L and project the light onto the surface P of an object in detection, the other part of which is light receiving optical fibers F2 which pick up the reflected monochromatic light from the surface P of an object in detection, transfer the light, and expose the light onto a photodetector D which produces correspondingly an electrical signal the voltage of which is measured for determination of the distance d between the object in detection and the optical fiber terminal head X. FIG. 4B illustrates a typical performance curve associated with the afore-said system, in which the front part of the curve contains a linear segment, within which the photoelectric voltage is linearly proportional to the distance d between the optical fiber terminal head and the object. This range of linearity is the section ST on the abscissa in FIG. 4B and point S is approximately 20 .mu.m to object (0) and point T is approximately 120 .mu.m to object (0). The center M of the section ST is taken as an operational center point. OM is the working distance that the optical fiber head shall be mounted in front of the object and is approximately 70 .mu.m. Since this mounting distance is so short, the optical fiber detecting head as well as the object in detection may be damaged due to mutual contact during installation or operation.